Mods
On the 16th of March, Jonas released a build that could be modded.
On the 17th of March 2022, two discord users known as Mandar1jn and ConfiG tried to mod the recently uploaded build, but the tools used (Undertale Mod Tool) had a problem so they set out to fix it. Around two hours later they had a fix, although not yet public, that made modding possible. This was announced with this tweet.
Jonas Tyroller's stance on modding is also known. He likes the capability and would like it to get a thriving community, but does not want to get involved too much. He does want to help promote cool mods. Currently, as of March 21 2022, there are 3 people with a direct line to Jonas Tyroller related to modding.
List of mod tools
Undertale Mod Tool
Undertale mod tool is a general Game Maker Studio 2 editor made by these people, originally it did not work with Will You Snail, but Mandar1jn and ConfiG fixed that.
Download here (must be logged into Github)
List of mods
Snailax
Snailax, its name a portmanteau of Snail Hax (Slang for hacks), is a level editor for Will You Snail? by Discord user MissingTextureMan101. Originally, it was made for the demos, but it is now compatible with the full game.
Volleyball with yourself
This mod by Discord user Pan2 replaces Helpy with a mirror snail so you can play Volleyball with yourself.